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The meteor that streaked across the Russian sky startled scientists worldwide, and prompted calls for greater vigilance to combat risks from the heavens.

Scientists at the US space agency Nasa estimated that the amount of energy released in the atmosphere above the Chelyabinsk region in the Ural Mountains was about 30 times greater than the force of the nuclear bomb dropped on Hiroshima during the second world war.

Russian divers scoured the bottom of a Chebarkul lake yesterday for fragments of the meteorite. But the six divers who searched its waters for three hours were able to finding nothing but mud and silt.

The emergencies minister stressed that no meteorite fragments had been discovered anywhere in the region so far, despite some 20,000 rescuers and recovery workers being dispatched to the region.

Officials said 2,962 buildings were damaged, with an estimated repair bill of 1 billion rubles (HK$255 million). Regional governor Mikhail Yure promised to have all the broken windows replaced within a week. But that is a long wait in a frigid region where the midday temperature was minus 12 degrees Celsius.

US space scientist Dr Bill Cooke said the orbital path of the meteor showed that it was an asteroid, not a comet.

It hit on the same day that astronomers were watching another larger rock, known as 2012 DA14, miss earth by just 28,000 kilometres, leaving experts scrambling to understand the two rare cosmic events.

Some initially speculated that earth was passing through a swarm of asteroids, but the Russian meteor came from the other direction.

"There is no relation there," said Nasa scientist Dr Paul Chodas. "We would expect an event of this magnitude to occur once every 100 years on average."

The meteor exploded about 24 kilometres above the earth's surface, causing a shockwave that triggered the global network of devices that listen for nuclear tests.

A UN team met last week to come up with recommendations on how best to track these orbiting space rocks.

"The goal is that we can see objects this size about two days before they hit the earth," said Dr Detlef Koschny, a scientist at the European Space Agency, referring to the Russian meteor. An ESA project aims to detect every object about to hit the earth, no matter what size.

"Then we can decide in each case: If it is just one metre, you could watch the spectacle," he said. "If it's bigger than five metres, you should step away from the window because the pressure might destroy it."

Russia said it would push ahead with efforts to "shoot down objects of alien origin", Deputy Prime Minister Dmitry Rogozin said on Twitter.

Giant impacts have changed the course of life on earth, notably 65 million years ago, when an object several kilometres wide came down off the coast of Mexico and killed off the dinosaurs.

"If we're lucky, none of us will see an asteroid coming towards the earth in our lifetime," said Dr Sergio Camacho, the head of the UN team. "But if we're not lucky and we didn't do anything, the only thing we might be able to do is evacuate."

Agence France-Presse, The New York Times, McClatchy Tribune, Bloomberg